1. Field of the Invention
The present invention relates to an image forming apparatus which ejects ink liquids of colors from a plurality of ink heads to form an image on an image forming medium, and relates to a positioning structure of an image forming apparatus in which displacements of superimposed colors by shifts of position of an image forming medium conveyed under a plurality of ink heads are eliminated.
2. Description of the Related Art
An image forming apparatus is comprised of the following image forming system. In the image forming system, a plurality of ink heads ejects ink liquids of different colors, that is, ink liquids of colors of black (K), cyan (C), magenta (M), and yellow (Y) to form an image on an image forming medium. The image forming apparatus comprises a conveying mechanism which conveys the image forming medium.
Four ink head groups which ejects the respective ink liquids of the colors of KCMY are provided above the conveying mechanism. Each of these ink head groups has a plurality of ink heads for the respective colors of KCMY. Therefore, a plurality of ink heads form one ink head group. The respective ink head groups are provided at predetermined intervals along a conveying direction of the image forming medium.
The conveying mechanism applies tensile forces, for example, to an endless band-shaped belt by a plurality of rollers, for example, three rollers to support the belt. In the conveying mechanism, a conveying-in side of the image forming medium is upstream, and a conveying-out side is downstream. For example, two rollers are provided on each of upstream and downstream sides. The remaining roller applies the tensile force to the belt. The roller on the downstream side is rotated/driven by a motor and the like. The roller on the upstream side moves following movement of the belt.
Therefore, when the roller on the downstream side is rotated/driven in the conveying mechanism, the belt moves. The image forming medium is laid on the belt, and conveyed by the movement of the belt. This conveying mechanism will be hereinafter referred to as a belt platen.
A pair of introduction rollers (hereinafter referred to as resist rollers) are provided on the conveying-in side (upstream side) of the belt platen. The resist roller is rotated/driven to introduce the image forming medium into the belt platen. Moreover, the resist roller regulates and corrects a posture such as a tilt of the image forming medium with respect to a conveying direction of the conveying mechanism.
In this image forming apparatus, the image forming medium is conveyed into the belt platen by a conveying force by the resist roller. When the image forming medium is conveyed into the belt platen, the image forming medium is adsorbed/held on the belt, and conveyed under the plurality of ink head groups with the movement of the belt. In this case, the resist roller freely rotates with respect to the image forming medium by a clutch function or the like. Accordingly, the image forming medium is conveyed under the plurality of ink head groups only by the conveying force by the belt platen.
The plurality of ink head groups ejects the respective ink liquids of KCMY. The respective ink liquids of KCMY are shot on the conveyed image forming medium. Accordingly, a full-color image is formed on the image forming medium.
A conveying direction of the image forming medium whose direction is regulated by the resist roller needs to agree with that of the image forming medium conveyed by the belt platen. If they do not agree with each other, positional shifts occur in color superimposition of the respective colors on the image forming medium at a time when the ink liquids of the respective colors are shot on the image forming medium to form the image.
For example, a technique concerning a recording apparatus such as a printer is described, for example, in Jpn. Pat. Appln. KOKAI Publication Nos. 2000-62995 and 2000-158637. It has been described in Jpn. Pat. Appln. KOKAI Publication No. 2000-62995 that a gap between a recording head and a sheet material is maintained constant, for example, regardless of a thickness of the sheet material as the image forming medium. In the Jpn. Pat. Appln. KOKAI Publication No. 2000-158637, a technique has been described in which recording paper can be securely prevented from being raised, and is stably discharged.